Optical recording media a which are designed to record information signals thereon by changing the magnetization direction of a perpendicularly magnetizable film in a recording layer and by optically reading out the recorded information signal s as is done similarly to magneto-optical disc have heretofore been proposed.
Such an optical pickup apparatus for reading out information signals recorded on an optical recording medium converges a linearly polarized light beam through an objective lens to irradiate the signal recording surface of the optical recording medium with the converged light beam and to detect changes in direction of the linear polarization of a returning light reflected by the optical recording medium by means of a photodetector. That is, the returning light f tom the optical recording medium has changed the polarization direction due to Kerr effect depending upon the magnetization direction at an area which is irradiated with the light beam. The differences of the magnetization direction of the optical recording medium are detected by detecting the changes in polarization direction of the returning light by detecting means such as a photodetector disposed in the optical pickup apparatus for detecting the strength of light, for example, through a beam splitter having a polarization dependence. In the magneto-optical reproducing system using the optical pickup apparatus, reproducing of international signals recorded on the optical recording medium is performed based upon the difference of the detected magnetization directions.
The magneto-optical disc is formed in such a manner that information signals are recorded by successively disposing recording pits on a recording track formed in a circumferential direction of the disc. The recording density of the information signals of such kind of magneto-optical disc is determined by the recording line density which is the recording density of recording pits successively arranged-on the recording track and the recording track density which is the pitch between recording tracks adjacent in a radial direction of the disc. The recording line density and the recording track density which determine the recording density of the information signals in the magneto-optical disc is restricted by the cut-off frequency f.sub.c by the optical pickup apparatus which reads the information signals from the magneto-optical disc. For example, if the spatial frequency of the changes in magnetization direction of the recording pits successively arranged on the recording track becomes higher than the cut-off frequency f.sub.c, read out of the information signals from the returning light becomes impossible. If the spatial frequency corresponding to the pitch between the recording tracks adjacent in a radial direction of the disc becomes higher than the cut-off frequency f.sub.c, so-called cross-talk in which information signals are read out from the adjacent recording track will occur.
The cut-off frequency of the optical pickup apparatus is determined by the numerical aperture NA of an objective lens forming the optical pickup apparatus and the wave length .lambda. of the light beam emitted from the optical pickup apparatus. The diameter, that is, the size of the beam spot of the light beam converged upon the signal recording surface of the magneto-optical disc is determined by the numerical aperture NA of the objective lens and the wave length .lambda. of the light beam. From these relations, the cut-off frequency f.sub.c of the optical pickup apparatus will be increased as the diameter of the beam spot of the light beam converged upon the optical recording medium is decreased. The cut-off frequency f.sub.c has a relation with the numerical number NA and the wave length .lambda. of the light beam represented by a first formula as follows: EQU f.sub.c =2NA/.lambda. (1)
In an optical pickup apparatus in which the numerical aperture NA of the objective lens is 0.5, and the wave length .lambda. of the light beam emitted from the optical source is 0.78 .mu.m in the formula 1, read out of the information signals recording at a spatial frequency up to 1280 lines/mm is possible. Read out of the information signals recorded at higher frequency is not possible. From the formula 1, it will suffice to increase the numerical number NA of the objective lens or to decrease the wave length .lambda. of the light beam in order to increase the cut-off frequency f.sub.c of the optical pickup apparatus. The recording density of the magneto-optical disc which can be readable by the optical pickup apparatus can be enhanced by increasing the cut-off frequency f.sub.c.
However, if the numerical aperture of the objective lens is increased, for example, compensation for aberration cannot be sufficiently achieved and it becomes impossible to design and manufacture a lens having excellent performances. Shortening the wave length of a light beam emitted from a semiconductor laser element commonly used as a light source makes it impossible to design and manufacture the semiconductor laser element since light having a shorter wave length has a higher energy of light.
Therefore, the present invention is proposed in view of the above-mentioned circumstances.
It is an object of the present invention to provide an optical pickup apparatus which is capable of reading out information signals recorded at a high recording density having a spatial frequency higher than a cut-off frequency determined by the numerical aperture of an objective lens and the wave length of a light beam without increasing the numerical aperture and the wave length.
It is another object of the present invention to provide a magneto-optical reproducing system which enables an optical information disc on which information signals are recorded at a high recording density to be used as a recording medium.